The quantum mechanical fluctuations in the axion field
during inflation are not the only source of perturbations
in the axion fluid in case inflation occurs after the
PQ phase transition. A second source is the effect on the
axion fluid of the adiabatic perturbations in the primordial
plasma [1]}. For the wavevectors of interest to us,
this second source of axion fluid perturbations dominates.
Indeed we find that the adiabatic perturbations in the
primordial plasma resonantly excite the axion field. As
far as we know, this effect has not been anticipated or
commented on until now.
